Several methods of detecting airborne contaminants are available, including gas chromatography, Fourier transform infrared spectroscopy, ionization techniques, electrochemical techniques, and adsorption by means of a charcoal canister. These methods require either very expensive equipment, or for the less expensive methods, results are either inaccurate or not available realtime. For example, in the past, charcoal detectors have been used to detect organic airborne contaminants in the atmosphere such as air pollutants, toxic gases, outgassing products, etc. In that method, the charcoal materials absorbed the contaminant, and the charcoal was later analyzed. The difficulty with the charcoal detection method was that the results were obtained after the fact. In other words, the results of contamination detection were delayed.
Recently, the use of quartz crystal sensors has been suggested as a means of sensing contamination in the atmosphere but the accuracy of sensing has been limited by uncertainties due to not knowing the resonator's temperature. For example, the frequency of the resonator may change by about 1 part per million per degree C. This means that if the output frequency of a sensor changes by 0.1 part per million, it could be due either to contamination or to a temperature change of 0.1 degree C.